Organic thin film transistors using organic semiconductive materials have been actively studied in recent years.
The organic semiconductive materials can be formed into a thin film by a simple wet process, such as printing and spin-coating. Therefore, they have advantages over thin film transistors using the conventional inorganic semiconductive material, such as the reduction in temperature for production processes.
Since use of the organic semiconductive material can reduce the temperature of the production processes, the thin film thereof can be formed on a plastic substrate which has generally low heat resistance. As a result, weights or costs of resulting electronics devices such as a display can be reduced, and various uses and applications thereof taking advantage of the flexible plastic substrate can be expected.
Some organic semiconductive materials have been proposed so far, such as poly(3-alkylthiophene) (see NPL 1), and a copolymer of dialkylfluorene and bithiophene (see NPL 2).
Since these organic semiconductive materials have some solubility to a solvent, though it is low, they can be formed into a thin film by coating or printing without using a technique such as vacuum deposition.
However, these polymer materials have restrictions in their purification methods. Therefore, some problems still remain. For example, it is complicated and time consuming to obtain a material of high purity, and quality of the material is not stable as there are variations in molecular weight or molecular weight distribution thereof.
Organic semiconductive materials of low molecular weight have also been proposed, such as acene materials (e.g. pentacene) (for example, see PTL 1).
It has been reported that the organic thin film transistor including an organic semiconductive layer formed of the aforementioned pentacene has relatively high electron mobility. However, these acene materials have extremely low solubility to common solvents. Therefore, these materials need to be vacuum-deposited to form a thin film thereof as an organic semiconductive layer of an organic thin film transistor. For this reason, these materials do not meet the demand in the art, which is to provide an organic semiconductive material that can be formed into a thin film by the aforementioned wet process such as coating or printing.
Moreover, there are some reports regarding low-molecular-weight organic semiconductive materials that have solubility to solvents. However, these materials still have problems. For example, a film formed of such material by a wet process is in the state of amorphous, and thus it is difficult to form a continuous film using such material due to crystal properties of the material. Moreover, properties of resulting elements formed of such material may vary, and desirable characteristics of the film cannot be obtained using such material.
In the case where the material has a crystal structure in which molecules are stacked by π-π interaction, such as the case of the dithienobenzothiophene derivative (see PTL2 and NPL 4), the crystals thereof tend to form needle shapes, and thus it may not be able to form a continuous thin film. In addition, the anisotropy of the charge transferring property may vary even within one crystal, and resulting elements formed using the material may largely vary. Accordingly, this material is not suitable for practical use. Especially, it should be noted that the crystal structure of the material (including the shape of the crystal) is difficult to assume based on the molecular structure of the material. Therefore, further improvements of organic semiconductive materials are still desired.